AustralIan said:
Also the 3A Geospark quoted for the field power is...shared with the embedded IO of the processor, the outputs having a combined max of 3A.
Yes, and no. I have already mentioned that the field power (FP) current is shared between embedded and local I/O...
Geospark said:
...provided the total current that the embedded I/O and local expansion modules draw
does not exceed both the available POINTBus backplane current of 1 A (@ 5VDC), and the field power current of 3 A (@ 24VDC).
However, this is "Field Power" to power the "I/O" devices connected to either the embedded or local expansion I/O and not just "output" power. The 3 A rating can include field power connected to both input and output devices. Example - 24 VDC supply (V/C) to a 3-wire inductive sensor where the signal wire is connected to an embedded or local input point. Or, indeed, 24 VDC negative (C) connected to an indicator lamp which is activated from an embedded or local output point. All adding to the consumption of the 3 A FP rating.
Further, the field power FP+ and FP- terminals on the controller are not linked to the VDC+ and VDC- terminals, which supply the processor and POINTBus backplane for the embedded and local expansion I/O. The FP terminals must be supplied separately.
This bit is conditional (or down to the user)...
For the initially released hardware Series A L1 controllers (L16ER, L18ER(M)) we are instructed to use a separate power supply for the controller and field power. In other words, do not wire a single power supply to the VDC terminals and loop to the FP terminals (I'm guilty of it). The reason for this is because the Series A controller's internal power supply was designed as non-isolated. As such, and when the supply terminals have been looped, the controller/backplane would be susceptible to any electrical noise or EMC that might be induced through the field power wiring.
I remember the day I first got my hands on an 5370 L16ER and then went on to read this fascinating design "feature". Two power supplies for a small packaged controller? These comedians never run out of new material, do they? To be honest, I've personally never followed this rule for the few L1's I've installed. But my applications were fairly simple with all discrete I/O devices close by and quite low current consumers. So I'm not advocating ignoring this advice, for everyone. In fact, for some applications it may be preferable to supply the controller and field power separately.
They seemed to have seen some sense though when designing the Series B hardware for the L1 controllers (L16ER, L18ER(M), and newly introduced Series A L19ER). The internal power supply is now isolated from the field power connections which means that we can now connect a single suitable power supply to both the VDC and FP terminals, as there is a greater degree of noise immunity. We can also still choose to wire separate power supplies to these terminals, if we wish.
An important fact to be aware of is that the 3 A rating for the field power is fixed, regardless of the output current made available at the power supply connected to the FP terminals. This is due to the fact that the field power is supplied to the embedded and local I/O V/C terminals through an internal and non-replaceable 3 Amp fuse. So for the field power "calculation", there really is none (or should be none).
As long as you choose a suitably rated power supply to meet or exceed the 3 A field power rating, you should not have to worry about calculating the initial or potentially future current draw. If using a single power supply for the controller/backplane and the field power, then we could take all the guesswork out of this and select a model rating to meet both combined. We know the field power rating is 3 A, so that just leaves the controller...
The power rating for the L1 controller embedded power supply is 30 VA, which is 1.25 A @ 24 VDC. A typical 50 W 2.1 A @ 24 VDC power supply would suit if just connecting the controller power separately, but we are looking to combine this with the field power into a single power supply. So we are roughly looking at say a minimum 5 A rated power supply to power everything. So something like the 1606-XLE120E 120 W 5 A power supply selected in the IAB example would more than suffice.
While not having to use two separate power supplies for the Series B (or A if you choose) L1 controllers, I would still recommend a dedicated power supply just for the controller and field power. All other general 24 VDC consumers for the installation would be best sourced from a separate power supply, typically a 10 A or 20 A model, depending upon application size.
Only if using a power supply of a rating less than the 3 A FP fuse rating would you need to consider calculating the FP I/O current draw. But I would argue, why scrimp here? Just throw the necessary or an overrated power supply at your L1 controllers, and walk away content.
It's also good practice to fuse after the power supply and before the FP terminals with a (from the manual)...
User-provided 4…5 A @ 3.15…5.5 A2t fuse.
Regards,
George